Various types of machines employ a wheel assist drive to provide all-wheel driving in slippery conditions. A motor grader, for example, is typically used in off-road environments to perform ditch work, site preparation, and other surface contouring and finishing tasks where obtaining sufficient traction may be difficult. The motor grader will often have a first set of wheels (such as a pair of rear wheels) driven directly by a combustion engine or primary hydraulic pump. A second set of wheels (such as a pair of front wheels) are typically used for steering. The front wheels, however, may be driven by hydraulic assist motors that are part of a wheel assist drive that permits all-wheel driving.
The wheel assist drive may be selectively engaged so that the machine operates in all-wheel drive mode only when desired. For example, the machine may include a user interface that allows an operator to switch the wheel assist drive on or off. Additionally, or alternatively, a controller operably coupled to the wheel assist drive may automatically engage or disengage the wheel assist drive based on feedback indicative of the level of traction between the wheels and the surface. In some embodiments, a free wheeling main valve may be utilized to block fluid flow to front wheel assist motors during when the wheel assist drive is not engaged during rear-wheel drive mode and to allow fluid flow to the front wheel assist motors when the wheel assist drive is engaged for all-wheel drive mode.
Some previous wheel assist drives have also been used in conjunction with a flow dividing valve to reduce front wheel slip during all-wheel drive mode. Without the flow dividing valve, when a first assist motor associated with a slipping wheel turns more rapidly, it draws an increased flow of hydraulic fluid. The increased flow to the slipping assist motor reduces hydraulic fluid flow to a second assist motor, which in turn reduces the rotational speed of the wheel that has traction. In excessive slip situations, substantially all of the hydraulic flow may be directed to the assist motor associated with the slipping wheel, thereby impacting the ability of the machine to travel over the surface. The flow dividing valve addresses the wheel slip condition by regulating hydraulic fluid flow to the two hydraulic assist motors coupled to the front wheels. In general, the flow dividing valve will regulate hydraulic pressures delivered to the assist motors by bringing those pressures closer to being equal. More specifically, the flow dividing valve senses hydraulic fluid pressures delivered to each of the hydraulic assist motors and, when a reduced pressure is sensed at one of the assist motors, the flow dividing valve adjusts to reduce hydraulic fluid flow to that assist motor, thereby improving traction of the wheel associated with the other assist motor.
U.S. Pat. No. 4,276,896 to Hunck et al. proposes a direction sensing valve that operates to maintain equal flow of fluid in the first and second motors in either forward or reverse direction of operation. While this valve may provide benefits, it does not provide all-wheel drive, anti-cavitation features and other desired features. A better valve is desired.